This invention relates generally to the field of optical comparators wherein an object, such as a manufactured part, is compared with a standard object to determine if differences exist therebetween. In particular, the invention relates to an optical comparator of simple inexpensive design especially suitable for testing complex assemblies such as printed circuit boards.
The use of optical comparators to determine whether a manufactured part meets certain specifications is well known. In the typical commercial optical comparator, the part being tested is illuminated and a shadow thereof is projected on a screen, the shadow being brought into coincidence with an outline of a master or reference part engraved or otherwise formed on the screen. In this manner, an observer can check to see if the part being tested deviates in any way from the master.
An optical comparator of the above-described type requires the observer make a careful examination of the projected image to determine whether a disparity exists. Such comparators are unsuitable for testing manufactured parts which are highly complex, such as printed circuit boards containing hundreds of electronic components.
In a printed circuit board providing a miniaturized electronic system, a conductive metal pattern, usually produced by an etching technique, is so laid out on an insulating panel as to make the necessary connections between resistors, capacitors, transistors and other circuit components. Connections between the leads of the components and the conductive pattern are made by inserting these leads into holes going through lands or pads in the conductive pattern, these connections being soldered by a dipping technique in which all of the soldered joints are formed at once.
The absence of a single component from the printed circuit board renders the board unacceptable. But with conventional optical comparators, a missing component is extremely difficult to detect by visual observation. Thus when comparing a master printed circuit board having hundreds of components scattered over the board with a manufactured board, the omission of a single tiny resistor can easily be overlooked.
To solve this problem, it is known to use optical comparators which are designed to call attention to missing details so that one can readily pick-out and identify distinctions between the master and the object being tested.
Thus in U.S. Pat. No. 3,744,917 the master and the test object are placed in side-by-side relation, such that when viewed through a mirror system, the master and test object are caused to optically fuse, the master and the test object being viewed by respective eyes of the observer. A rotating shutter intermittently interrupts the light path to each eye so that the fused images are actually viewed successively rather than simultaneously and differences between the compared objects appear as pulses indicative of the missing detail.
Similar optical comparators of the switching-type making use of a rotating shutter mechanism to alternatively view a master and a test object are disclosed in the IBM Technical Disclosure Bulletin, Vol. 10 No. 11, April 1968 (pages 1661 and 1662) and in Vol. 13, No. 12, May, 1971 (page 3630). In these comparators, if the object and the references are identical, the viewer sees a fixed image, but if a difference therebetween exists, a readily noticeable flicker is produced.
While image-switching optical comparators of the type heretofore known are effective in picking out missing details, they involve motors, rotary shutters and complicated mirrors, prisms, and other optical arrangements. As a consequence, such comparators are relatively cumbersome and costly.